1. Field of the Invention
The present invention relates to a receiver used in radio communication and more, particularly, to a receiver in which intermodulation caused by a plurality of interfering waves synthesized by a non-linearity of a receiver is effectively suppressed, when two or more intense interfering waves (other than) a signal being received by the receiver, exist at the same time.
2. Description of the Prior Art
In a receiver used in a radio communication equipment, such as a cellular phone, a radio selective calling receiver (a so-called pager) or the like, two or more intense interfering wave signals (other than the desired wave signal being received by the receiver) may exist at the same time. These interfering wave signals are synthesized by a non-linearity of a high-frequency amplifier and a frequency mixer constituting the receiver, and may fall to a frequency of the desired wave signal as the case may be. This problem is usually known as a cross modulation or an intermodulation (hereinafter abbreviated as IM), which is an important factor influencing the performance of the receiver. Upon generation of an IM signal, even when the receiver receives the desired wave signal sufficiently large relative to thermal noise, a receiving property of the receiver are deteriorated. This causes a deterioration of an error ratio in digital communication or a deterioration of SINAD (Signal+Noise+Distortion to Noise+Distortion ratio) in analog communication.
Here, methods of preventing the deterioration of the receiving property of the receiver caused by the IM signal are disclosed in a publication of patent application laid-open No. Hei 5-335857 (hereinafter, referred to as the first prior art) and a publication of patent application laid-open No. Hei 7-212262 (hereinafter, referred to as the second prior art).
FIG. 1 is a view showing a constitution of a receiver disclosed in the first prior art. In FIG. 1, a signal received by an antenna 11 is amplified in a high-frequency amplifier 13, passes a variable attenuator 15, and is filtered in a high-frequency (band pass) filter 17. The signal transmitted from the high-frequency filter 17 is branched into frequency mixers (converters) 19 and 21, and converted to base band signals by a local oscillating signal from local oscillator 23 and phase shifter (.pi./2) 25 (which provides a .pi./2 phase shift). Thereafter, the base band signals from the frequency mixers 19 and 21 pass base band filters 27 and 29, respectively, and are demodulated into digital signals via demodulator 31. A frame synchronous signal is detected in a control circuit 33. If the frame synchronous signal is not detected within a predetermined time in the control circuit 33, a step-out signal is transmitted from the control circuit 33 to a gain control means 35, and the gain control means 35 changes an attenuation quantity of the variable attenuator 15. By changing the attenuation quantity of the variable attenuator 15, influence of cross modulation caused by interfering waves is removed. When the frame synchronous signal is detected in the control circuit 33, the gain control means 35 operates so as to reset the attenuation quantity of the variable attenuator 15 to its original value. Here, for convenience of description, the frequency mixers 19, 21, the local oscillator 23, the phase shifter 25 and the base band filters 27 and 29 are called a base band signal conversion means 39. Also, the variable attenuator 15, the gain control means 35, the demodulator 31 and the control circuit 33 are called a control means 37.
According to the prior art, considering the attenuation quantity of the variable attenuator 15 beforehand, when a field strength of the received signal transmitted to the control circuit 33 is large and no frame synchronous signal is detected, then it is determined that cross modulation is caused in the signal band of a desired wave signal by a plurality of interfering waves other than the desired wave signal, and the attenuation quantity of the variable attenuator 15 is controlled. Therefore, a cross modulation signal generated after the variable attenuator 15 can be suppressed.
FIGS. 2, 3 and 4 are views showing a constitution of a receiver disclosed in the second prior art. In FIG. 2, a signal received by the antenna 11 passes a resonance circuit 39 (details of which are described later with reference to FIG. 3), and is transmitted via an attenuator 41 to an RF portion 43. The RF portion 43 generates a signal for controlling the attenuation quantity of the attenuator 41 in accordance with an intensity of the received signal, thereby controlling the attenuator 41 so that the field intensity of the received signal transmitted to the RF portion 43 is at a constant level. The received signal (controlled to the constant level) is amplified in the RF portion 43, converted from an analog signal to a digital signal by an analog-digital converter (A/D) 45, and subsequently transmitted to a digital mixer 47. The digital mixer 47 also receives a local oscillating signal from a frequency synthesizer 49. In the digital mixer 47, the digital signal from the A/D 45 is converted to an intermediate frequency in a known heterodyne process. A control portion 51 is provided so as to control not only a frequency emitted from the frequency synthesizer 49 but a resonance frequency of the resonance circuit 39 provided on a front end of the receiver. FIG. 3 is a view showing an inner construction of the resonance circuit 39 of FIG. 2. The resonance circuit 39 includes a parallel LC resonance circuit 57 having a variable inductor 53 and a fixed capacitor 55; and a parallel LC resonance circuit 63 having a variable inductor 59 and a fixed capacitor 61 are connected in series. The variable inductors 53 and 59 can have inductance values controlled by control signals from the control portion 51 (see FIG. 2), respectively. Resonance frequencies of the parallel LC resonance circuits 57 and 63 can be changed. Frequency properties of the parallel LC resonance circuits 57 and 63 are shown in FIG. 4. The frequency properties of the parallel LC resonance circuits 57 and 63 operate as known notch filters which cause attenuation of 50dB or more at respective resonance frequencies of the circuits, and are not substantially attenuated in a frequency band other than the resonance frequencies.
According to the prior art, when, two or more interfering waves exist (other than the desired wave), a field strength of the received signal is large and a receiving sensitivity is degraded, then it is determined that the interfering waves are caused by cross modulation in a band of the desired wave frequency, and the interfering waves are removed by adapting the resonance frequencies of the parallel LC resonance circuits 57 and 63 to the frequencies of the interfering waves. Therefore, deterioration of the receiving sensitivity due to the cross modulation can be prevented.
A first problem with the afore-mentioned first prior art system is that since the deterioration of the receiving sensitivity by the cross modulation caused by a plurality of interfering waves (other than the received frequency) is determined by detecting whether or not the frame synchronous signal exists in a received frame. Therefore, it cannot be determined whether or not the cross modulation is caused by a plurality of interfering waves.
For example, in North America, there is a system called TIA.IS95, which has a pilot channel transmitting a signal for obtaining synchronization with the system, a synchronous channel transmitting a signal for obtaining frame synchronization and a plurality of channels of a traffic channel for transmitting voice or other information signals. Further, a transmission power ratio of each channel from a base station is different in such a manner that the pilot channel: the synchronous channel: the traffic channel=3:1:1. In the system, a plurality of intense interfering waves which cause cross modulation may exist, and a cross modulation signal produced by the interfering may drop in a band of a desired wave frequency. In this case, even if a receiver can obtain synchronization with the base station via the pilot channel, at the time of receipt of the traffic channel, the cross modulation signal caused by two or more interfering waves drops in the received band of the desired wave frequency, and the receiving sensitivity might be deteriorated. This is because the receiver takes a frame synchronous signal from a received frame, and it is determined using detection of the frame synchronous signal that the deterioration of the receiving sensitivity by the cross modulation caused by a plurality of interfering waves occurs. For example, like the aforementioned North American TIAoIS95 system, in which a synchronous signal is exchanged with the base station using a plurality of channels, the receiver determines, only by determining whether or not the frame synchronous signal is detected, that the deterioration of the receiving sensitivity by the cross modulation caused by a plurality of interfering waves occurs.
A second problem with the afore-mentioned first prior art system is that using the variable attenuator 15 inserted in the front end of the receiver, not only the cross modulation signal produced by a plurality of interfering waves but the field intensity of the received signals including the desired wave signal are adjusted. Therefore, the input power level of the demodulation signal transmitted to the demodulator 31 is not constant, and the demodulator 31 requires a very large input dynamic range.
Reasons for this are as follows. When the cross modulation is caused by a plurality of interfering waves, to suppress the cross modulation in or after the variable attenuator 15, the attenuation quantity of the variable attenuator 15 is controlled to be large. However, at the same time the cross modulation caused by a plurality of interfering waves is suppressed, the received power level of the desired wave transmitted to the demodulator 31 is also suppressed, and the input power level of the demodulator 31 is decreased. Also, when no interfering waves causing the cross modulation exist, attenuation quantity of the variable attenuator 15 is controlled to be a small. Therefore, the received power level transmitted to the demodulator 31 is large as compared with when the interfering waves exist.
A first problem with the aforementioned second prior art is that when a plurality of combined interfering waves causing cross modulation exist, a plurality of notch filters which can vary the resonance frequencies as shown in FIG. 3 are required at the front end of the receiver. Also, even when a plurality of variable notch filters are provided, the resonance frequency of the resonance circuit has to be synchronized with each of the frequencies of the interfering waves causing cross modulation. Therefore, controlling the resonance circuit becomes very complicated.
This is because the frequencies of the interfering waves causing the cross modulation are usually supposed to be a three-dimensional distortion component produced by the non-linearity of the high-frequency amplifier constituting the receiver. When the frequency of the desired wave is set as fc and the frequency of the interfering waves causing cross modulation is an optional fl, then fc+fl and fc+2fl or fc-fl and fc-2fl can be provided. An infinite number of combinations of the interfering wave frequencies exist. For this, it is difficult to provide a plurality of notch filters which can be synchronized to all the interfering wave frequencies. Further, it is difficult to control and synchronize the resonance frequency of the resonance circuit to all the interfering wave frequencies.